Thermal interface material formulations generally consist of high thermal conducting fillers with inherent thermal conductivity. These fillers, when incorporated into a matrix over its percolation volume, lead to exponential increment in bulk thermal conductivity. The bulk thermal conductivity of a thermal interface material depends heavily on the volume percent loading of the fillers. As the amount of fillers increases, the interaction between filler and base resin increases. This leads to a dramatic jump in the viscosity of the formulation which causes difficulty in processing and dispensing the formulation. Dispersion aids or wetters are the class of material that reduce these interactions and help in efficient dispersion of fillers without significant viscosity build up.
U.S. Pat. No. 8,017,684 describes an alkoxy silane as a wetter wherein the alkoxy groups interact with the filler surface. Alkoxy silane may work as a wetter, but volatilizes from the formulation under the operating temperature due to its low molecular weight and viscosity. This leads to dry-out of the formulation. JP2005325212 and U.S. Pat. Nos. 6,306,957; 6,380,301; 7,291,671; 8,093,331; 8,119,758; and 8,383,005 discuss the use of hydrolysable methyl polysiloxanes as a dispersing aid. Compared to alkoxy silane, however, alkoxy functionalized polyorganosiloxanes have very poor wettability and must be added at high loadings. U.S. Pat. No. 7,622,539 describes acrylate functionalized alkoxy organopolysiloxanes, wherein the wetter molecule, after reacting with filler surface, can be cross-linked within the matrix to avoid volatilization of bleed out. This may affect cross-link density and eventually the elasticity of the cured matrix. U.S. Pat. Nos. 7,547,743 and 6,844,393 describe the use of vinyl functionalized alkoxy polysiloxanes for the same application. U.S. Pat. No. 7,692,032 describes both curable and non-curable compound of alkoxy functionalized cyclosiloxanes for thermal grease and rubber applications. Apart from this, JP2015209466 describes the application of β-ketoester functionalized polyorgano-metalosiloxanes wherein the metal-ligand interacts with the filler surface. U.S. Pat. No. 8,618,211 mentions a reference to the use of anionic, cationic, or non-ionic organic surfactants such as Tergitol and Triton-X as a wetter, but these will phase out of the formulation eventually.
Boron nitride is sought after to use as a filler because of its high thermal conductivity and low electrical conductivity. Additionally, it is a soft, lubricaious and low density filler, which is advantageous in dispensing and to provide a soft formulation. But unlike many fillers, it lacks any surface functionality, which makes it difficult to effectively disperse boron nitride in a thermal formulation. Traditional wetters with alkoxy functionalization are not sufficient to use boron nitride as it lacks surface interlinking with boron nitride surface. Efforts to surface modify boron nitride have yielded limited results with low success.